The present invention generally relates to a focus control method and an apparatus therefor. More particularly, the present invention is concerned with a focus control method and an apparatus therefor which can advantageously be employed in an optical recording system (which may also be referred to as the exposure system) for optically recording data on a medium such as an optical disk or the like. Further, the preset invention is concerned with the optical recording system itself as well.
In the exposure equipment or optical writing system for optically writing or recording data on a medium such as a disk or the like, a focus control apparatus is employed for maintaining constant the distance which intervenes between a disk (i.e., object, hereinafter also referred to as the raw disk) and an objective lens. For effectuating the focus control, a focusing light beam having a wavelength differing from that of a recording or writing light beam is employed for protecting the raw disk against the influence exerted by the focusing light beam. Heretofore, a so-called achromatic lens whose focal position or focal length remains the same for different wavelengths has been used as the objective lens for the purpose of focusing the recording light beam. Thus, both the focusing light beam and the recording light beam can impinge onto the objective lens in the form of parallel light beams without incurring any appreciable degradation in the focusing precision or accuracy even when the optical path length changes due to upward/downward movement or displacement of the objective lens following the change in the position of the raw disk.
As one of the related art, there may be mentioned, for example, the system which is disclosed in Japanese Patent Application Laid-Open Publication No. 73491/1995 (JP-A-7-73491).
With the conventional system such as disclosed in the above-mentioned publication, an achromatic lens is used as the objective lens. Accordingly, even when the wavelength of the recording laser beam differs from that of the focusing laser beam, there makes appearance no difference in the focal length due to the different wavelengths.
However, for the recording light beam of a wavelength in the deeper or shorter ultraviolet range for systems of the next generation, the achromatic lens is not available as the objective lens yet. In general, in order to make the position of the focal point of the recording light beam incident on the objective lens as the parallel light beam coincide with that of the focal point of the focusing light beam in the case where the achromatic lens is not employed as the objective lens, it is required to cause the focusing light beam to be incident on the objective lens in a non-parallel state in consideration of refraction ascribable to the difference in the wavelength. As a consequence of this, when the optical path length of the focusing light beam changes due to upward/downward movement or displacement of the objective lens in following the change in the position of the raw disk, the position of the focal point of the focusing light beam undergoes change, giving rise to a problem that the focusing precision or accuracy becomes degraded.
More specifically, referring to FIG. 1 of the accompanying drawings, it is assumed that a commercially available objective lens 3 (rated for ca. 250 nm) is employed in combination with a convex lens 12 (having a focal length f=2.0 mm) with the inter-lens distance of 150 mm between the objective lens 3 and the convex lens 12. In this optical system, when the raw disk 4 moves downwardly or descend by 1 xcexcm, being accompanied with downward movement of the objective lens 3 by 1 xcexcm in following the displacement of the raw disk, i.e., when the optical path length increases by 1 xcexcm, the position of the focal point of the focusing light beam will deviate from that of the recording light beam by ca. 40 nm. Since the flatness of the raw disk 4 is on the order of 10 xcexcm according to the standard specifications, there will arise deviation on the order of 400 nm at maximum between the position of the focal point of the recording light beam and that of the focusing light beam, which is comparable to the depth of focus for the recording light beam of wavelength in the deeper ultraviolet range, thus exerting adverse influence to the focusing accuracy or precision.
In the light of the state of the art described above, it is an object of the present invention to provide a focus control method which is capable of performing the focus control with an enhanced accuracy without need for employing an achromatic lens as the objective lens.
Another object of the present invention is to provide a focus control apparatus for carrying out the method mentioned above.
It is a further object of the present invention to provide an exposure or optical recording system which incorporates the above-mentioned focus control apparatus.
In view of the above and other objects which will become apparent as the description proceeds, it is taught according to a general aspect of the present invention that change in the optical path length of the focusing light beam is detected by detecting upward/downward movement(s) of the objective lens to thereby control the movement of the objective lens so that the change in the optical path length of the focusing light beam can correctively be compensated for.
Thus, there is provided according to an aspect of the present invention a focus control method in which the change in the optical path length of the focusing light beam is detected by detecting upward/downward movement(s) of the objective lens for thereby driving the objective lens in such a manner that the focused state of the focusing light beam can be sustained or maintained.
Further, there is provided according to another aspect of the present invention a focus control apparatus in which a focusing light beam is used and which includes a unit for detecting change of the optical path length of the focusing light beam and a unit for correcting the position of an objective lens on the basis of the change as detected.
Furthermore, there is provided according to yet anther aspect of the present invention an optical recording system equipped with the focus control apparatus which includes a unit for detecting change of the optical path length precision of the focusing light beam and a unit for correcting the position of an objective lens on the basis of the change as detected.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.